Due to the increasing energy demands, offshore oil and gas production is moving into deeper waters. For ensuring an efficient and secure production, processing facilities are being installed at the ocean floor. Such subsea installations can comprise a range of components, including pumps, compressors and the like as well as a power grid for operating them. The power grid may for example comprise a subsea transformer, subsea switchgear and subsea variable speed drives. The components of the subsea installation need to be protected from the surrounding sea water, in which pressures of 300 bar or more can prevail (at installation depths of 3.000 m or more).
Two solutions were proposed for dealing with these high pressures. A pressure resistant enclosure can be provided, which has a close to atmospheric internal pressure, enabling the use of conventional electric and mechanical components. Such enclosures need to have relatively thick walls and are thus bulky and heavy, since they have to withstand high differential pressures.
Another solution is the use of pressurized (or pressure compensated) enclosures, which comprise a volume/pressure compensator which balances the pressure in the enclosure to the pressure prevailing in the ambient sea water.
The pressurized enclosure is generally filled with a liquid, and components operated inside the pressurized enclosure are made to be operable under high pressures. The pressure/volume compensator compensates for variations in the volume of the liquid filling the enclosure, which may occur due to variations in outside pressure and/or temperature. Temperature changes can be cause by internal heating, e.g. due to electric losses.
Pressure compensators may include metal bellows, rubber bellows, pistons or the like. Bellows can have the disadvantage that they are either expensive to produce, or their configuration is such that the stroke length of the bellow is limited. In the latter case, a pressure compensator for a large volume of liquid also requires large volume. For some types of bellows, the bellow needs to have a size of more than three times of the size of the compensated volume. This results in a low utilization factor of the volume of the compensator system. Furthermore, the liquid filling such pressure compensator needs to be compensated itself. Such compensator systems can thus be relatively large and heavy.
In the document WO2010/034880A1, a pressure compensator is disclosed which has a first bellows chamber that is surrounded by a second bellows chamber, the second bellows chamber forming a closed intermediate space around the first bellows chamber. A double barrier against the ingress of sea water is thus obtained, but the pressure compensation capacity is the same as if the first bellows chamber was provided by itself.
The document WO2011/088840A1 discloses a pressure compensation system which achieves a double barrier against the ingress of sea water.
It is desirable to provide a pressure compensator for use with a subsea device that can be manufactured easily and cost efficiently. It is further desirable that the pressure compensator provides security during operation and has a long lifetime. It is desirable to reduce the size of the pressure compensator, and to increase the utilization factor.